Pre-performance routines commonly include stretching and intense muscular contractions in an attempt to optimise muscular performance and reduce injury risk. However, the isolated and combined effects of stretching and muscle contractions on neuromuscular performance are not well described. The aims of this research were to examine the effects of acute static stretch and intense muscular contractions on force production of the human plantarflexors and to examine possible mechanical and neuromuscular mechanisms underpinning any changes. Techniques including isokinetic dynamometry, electromyography (EMG), sonography and motion analysis were used in three studies on recreationally active human volunteers (n=20). In the first study, three 60-s passive stretches was found to significantly reduce concentric plantarflexor joint moment (5.0%; P<0.05), which was correlated (r = 0.81; P<0.01) with a reduction in EMG amplitude (9.2%; P<0.05). No reduction in Achilles tendon stiffness or gastrocnemius medialis (GM) muscle operating length was found, and all measures recovered by 30 min. This indicates that post-stretch force losses are transient and are largely associated with reduced neuromuscular activity (EMG amplitude) rather than changes in the muscles’ operating lengths. Nonetheless, strong muscular contractions, commonly performed during pre-performance routines and incorporated into research designs, may influence the effects of stretch. In the second study it was found that six 8-s maximal isometric contractions reduced Achilles tendon stiffness (10.9%; P<0.01) and passive joint moment (4.9%; P<0.01) and also significantly reduced concentric moment (11.5%; P<0.01), which was again correlated (r = 0.90; P<0.01) with a reduction in EMG amplitude (21.0%; P<0.01). Importantly, a subsequent bout of static stretch, which was identical to that used in study 1, did not result in a further change in any measure (P > 0.05). Whilst concentric moment and EMG recovered 30 min later, the decreases in Achilles tendon stiffness and passive moment remained. Thus, the normal stretch-induced reductions in force production were removed when isometric contractions were performed prior to stretch, but this was because concentric strength and neuromuscular activity were already affected; the reduction in concentric moment without a decrease in isometric moment indicates a contraction mode-specific response. The final study revealed that the use of concentric contractions (6×8-s) also resulted in similar reductions in Achilles tendon stiffness (11.7%; P<0.01) and concentric joint moment (6.6%; P<0.01) as the isometric contractions, and these were correlated (r = 0.94; P<0.01) with a reduction in EMG amplitude (10.2%; P<0.01). However, a further reduction in concentric moment was detected following an identical bout of static stretch (5.8%; P<0.01) with no further change in EMG. Importantly, EMG recovered 30 min later while concentric moment remained depressed (9.2%; P<0.01), indicating a muscle-based mechanism for these force losses. No reduction in GM muscle operating length was found, removing this as a mechanism underpinning the losses in force. The findings from the present series of studies have important implications for research study design as the warm-up imposed on subjects prior to stretch seems to strongly influence the impact of stretch. Furthermore, the results also have important practical implications in the formulation of pre-performance routines where maximal force production in the plantarflexors is an important goal.
|Number of pages||151|
|Publication status||Published - 2010|